I love rack deadlifts myself. Do a HEAVY controlled set of these and I suspect you won't be thinking of them as an accesory exercise anymore. With a wide grip I find these really hit the lats in a similar fashion to rows, as well as working the traps and lower back very nicely. I find I can do these early in the week, and still be able to squat heavy later in the week. If I perform both full deadlifts and squats, my knees take a real pounding, so I have to pick one or the other.

It depends on the lifter as to whether you are prone to hurting your back doing them. I have pulled over 800 from well below the knees on a rack deadlift. I pull near 700 off the floor. I don't suspect I'll be throwing my back out anytime soon.

However, if you have a kid who can only deadlift 225 off the floor and he's pulling double that in the rack, then it's probably time to get a little stronger on the full range stuff first.

Silver is correct. It is a good exercise for lats, erectors, traps, etc., though honestly it is still inferior to full range pulls. But as long as you are doing full range pulls, then have at it with the rack pulls.

i have a bad left knee that i am rehabing... how much pressure do you think full deads put on them? i used to do sumo deads b/c i have really long legs/arms and i could lift more than regular style deads... now i just do rack deads to help the knees, is that ok? how much pressure is on the knees from sumo/reg deads?

With rack deads you should never go heavier than 10% of your 1 RM of a regular deadlift according to Westside training principles. Also, make sure that the bottom of the plate is only 8 inches off the floor (or less, 4" or 6" is ok too) when it's at rest on the rack. It's a waste of time and it's not going to help your regular deadlift to put the bar knee height.